Mechanics of Rugby Union Scrums: stability and collective production of force

Rugby Union is a contact sport played by two 15-players teams. During scrummaging, two 8-players groups called packs confront each other in a 3-4-1 configuration. The objective of each team is to push the opponent backwards while staying stable. A surprising fact is that there is a loss in collective force produced by the pack : 8 international level players in the pack able to generate together only 75% of the sum of forces produced individually.
We study experimentally both the stability of the scrum with a simple beam analogy, and the origin of the loss in collective force produced by players of Racing 92, a top level team of Top 14, the French national championship. We perform experiments at the « Atelier de la mêlée », the Rugby French Federation training tool to measure the forces produced by players and packs. By coupling forces measurements with a Particle Image Velocimetry (PIV) analysis of videos taken from above, we show that there is a correlation between force fluctuations and the lack of synchronization of the 8 players pushing together.